1. Field of the Invention
The present invention relates to full duplex communications terminals in general, and to the mitigation of ambient noise in such terminals in particular. More particularly still, it relates to mode or state control in handsfree or loudspeaking communications terminals in telephony but is not restricted thereto. For example, the invention is equally applicable to handset operation (as opposed to handsfree operation).
2. Prior Art of the Invention
The most basic loudspeaking communications terminal would employ two voice switched states: full receive or full transmit. A third quiescent or idle state would overcome the "dead" sensation to the far-end user at the remote terminal during speech inactivity, and reduces the degree of switching required between receive and remote terminal during speech inactivity, and reduces the degree of switching required between receive and transmit states, which improves performance subjectively.
For example, U.S. Pat. No. 3,889,059, issued June 10, 1975, to Thompson et al, discloses a speakerphone apparatus which has transmit and receive channels and also includes a control means for alternately operating the apparatus in transmitting and receiving modes. The gain in the transmitting mode is regulated in inverse relation to substantially monotonous amplitude sound, as distinct from voice-like impulse amplitude sound. The natural tendency of a user to speak more loudly in a noisy environment characterized by the presence of monotonous ambient sound, is thereby compensated for and a more constant speech signal level is presented to the transmission facility. In a voice controlled speaker-phone embodiment there is also an idle mode of operation in which the gain in the transmit channel is likewise regulated by the amount of substantially monotonous ambient room noise.
The ambient noise determination in the above patent is described as follows:
"In operation, the noise circuit in FIG. 2 provides the ambient noise voltage and current signals. The circuit responds very slowly to positive going signals and very rapidly to negative going signals so that at the output of the differential amplifier 35 a signal appears which is substantially related to the ambient noise in the operating environment of the apparatus. Here and in the remainder of the disclosure "noise" is taken to mean signals of a monotonous or generally steady nature as compared with typical speech envelope signals, which are well known to have an irregular impulse envelope characteristic. The output signal is substantially unaffected by speech content in the signal from the microphone signal rectifier 14. The transistor 38 operates as an emitter follower. The signal from the output of the differential amplifier 35 is connected to the base of the transistor 38 via the resistor 37 and hence a corresponding voltage signal appears at the emitter electrode of the transistor 38. This signal or noise voltage develops a linearly related current through the resistor 39, which then is available as a noise current at the collector electrode of the transistor 38." PA1 "To do this, the signal from each channel is applied to an analogue-digital signal envelope converter (30,31), which generates a multi-bit word representative of the current speech amplitude in its channel. Those words are applied to a comparator (32) whose output is indicative of which channel passes speech. Such a converter follows the envelope of the speech signal in its channel to produce the multi-bit word representing the current speech amplitude in its channel. The comparator's output goes via control logic (33) to two attenuators (34,35), one in each channel. Thus only the channel which is actually passing speech, or the one with the higher speech amplitude, is enabled."
In U.S. Pat. No. 4,555,596, issued Nov. 26, 1985, to Blomley, either the transmitting or the receiving speech channel is disabled: